Line a slice tin with tin foil. Either line the entire tin with a single sheet, or (pictured) line the tin in 3-4 rows so that each row can lift out individually. Lining in separate rows helps the fudge cool more rapidly.

Place the butter in a saucepan and melt over a medium heat.

Add the condensed milk. Use a wooden spoon to stir until combined. Cook, stirring, for 2 minutes to heat the mixture so that it will be able to melt the chocolate.

Turn off the heat. Add the chocolate pieces to the butter and condensed milk mixture and stir to combine. Continue stirring until all the chocolate is melted. If the chocolate is not entirely melted, place the pan over low heat and stir continuously until the chocolate is completely melted.

Pour the mixture into the prepared slice tin. Chill in the refrigerator for 1-2 hours or until set.